A steering system for steering a vehicle includes a pinion gear on the side of a steering shaft and a rack bar on which rack teeth are formed on the side of tie-rods which connect left and right front road wheels. Then, rotational steering force transmitted from a steering wheel is converted to a horizontal lateral force by a steering gear box, and the horizontal lateral force is then transmitted to the road wheels by way of the pinion gear and the rack teeth, whereby a rotational force around king pins is applied to the road wheels (see, e.g., JP2007-253190A).
On the rack bar, the steering force transmitted from the steering wheel is transmitted from contact faces of the pinion gear and the rack teeth. Consequently, the larger the contact faces are, the more efficiently the steering force can be transmitted.
In a process of producing such a rack bar, there is known a method for increasing the strength of a rack bar to be produced by attaining a uniform flow of a metal material into recess portions in a die during a press rolling of the material (see, e.g., JP2002-86243A).
The rack bar described above has the following problem. Namely, as a method for forming rack teeth, there is known a method for press forging a pipe member 310 from an interior 311 thereof, as shown in FIG. 7. In this method, the press forging is carried out by employing mandrels 340 having a tapered projection on an upper surface side as a mandrel 340. When a mandrel 340 is press fitted into the pipe member 310, a material of an upper surface 312 of the pipe member 310 is caused to project into recess portions in a teeth forming die 350 by the projection on the mandrel 340. Thereafter, the mandrel 340 is replaced with a mandrel 340 having a larger projection to be press fitted into the pipe member 310. This process is repeated, whereby a rack toothed portion 320 is formed on the upper surface 312 of the pipe member 310. In this process, the material of the upper surface 312 is rolled and forged, and therefore, large stress is produced particularly at portions of the recess portions of the teeth forming die 350 which transfer a profile of central portions of the rack toothed portion 320 with respect to a tooth width direction. Because of this, when the number of times of forming a rack toothed portion 320 is increased, minute cracks are produced in the teeth forming die 350. These cracks are produced particularly in the central portions where the large stress is produced. Then, the material enters the cracks to produce burs. As this occurs, since a rack bar produced from the pipe member 310 becomes a defective product, the teeth forming die 350 needs to be replaced with another which is free from such cracks.
FIG. 8 is a perspective view showing one example of general rack teeth in an enlarged fashion, FIG. 9 is a perspective view showing another example of general rack teeth in an enlarged fashion, and FIG. 10 is an explanatory diagram illustrating a comparison of sectional shapes of the rack teeth shown in FIGS. 8 and 9.
In FIG. 8, reference numeral 321 denotes one example of general rack teeth, 322 denotes a tooth tip portion of the rack tooth 321, 323 denotes a tooth tip rounded corner portion, 324 denotes a face or contact face, 325 denotes a tooth bottom rounded corner portion, and 326 denotes a tooth bottom portion. In addition, in FIG. 10, S1 denotes a sectional shape of the rack tooth 321 shown in FIG. 8, L1 denotes a face length (a straight length) of the contact face 324, and R1 denotes a radius defining the tooth tip rounded corner portion 323. The tooth tip portion 322 and the contact face 324 connects to each other along an R shape of the tooth tip rounded corner portion 323, and the contact face 324 and the tooth bottom portion 326 connect to each other along an R shape of the tooth bottom rounded corner portion 325. These connections by way of the R shapes result from forming the rack teeth 320 through forging and intended to increase the life of the teeth forming die.
As shown by S1 in FIG. 10, although when the radius R1 defining the tooth tip rounded corner portion 323 is large, the life of the teeth forming die can be extended, the straight length L1 becomes short. Since the contact face 324 is a portion where the steering force is transmitted directly from a face of each of teeth of the pinion gear, in case the straight length L1 is short, the area of the contact face 324 becomes small, and therefore, the durability and strength and the steering force transmission efficiency with the pinion gear of the rack tooth 321 become insufficient.
On the other hand, in FIG. 9, reference numeral 331 denotes another example of general rack teeth, 332 denotes a tooth tip portion of the rack tooth 331, 333 denotes a tooth tip rounded corner portion, 334 denotes a face or contact face, 335 denotes a tooth bottom rounded corner portion, and 336 denotes a tooth bottom portion. In addition, in FIG. 10, S2 denotes a sectional shape of the rack tooth 331 shown in FIG. 9, L2 denotes a face length (a straight length) of the contact face 334, and R2 denotes a radius defining the tooth tip rounded corner portion 333. The tooth tip portion 332 and the contact face 334 connects to each other along an R shape of the tooth tip rounded corner portion 333, and the contact face 334 and the tooth bottom portion 336 connect to each other along an R shape of the tooth bottom rounded corner portion 335.
As shown by S2 in FIG. 10, when the straight length L2 becomes long, the area of the contact face 334 becomes large, and therefore, the durability and strength and the steering force transmission efficiency with the pinion gear of the rack tooth 331 can be increased. On the other hand, when the straight length L2 becomes long, the radius R2 defining a rounded corner portion of the tooth tip becomes small, and the life of the teeth forming die becomes insufficient.